Electric wave filter



sept. 23, 1952 Filed Jan. 5, 1946 B. B. JACOBSEN ET'AL ELECTRIC WAVE FILTER 4 Sheets-Sheet 1 /O ELECTRIC, 1l

- WAVE i i L O3 Emmen ,7 VOLTAGE 'y {"M- m MULTIDLIER 'Bam' Bumw Incensen. n Magnan UoNN Hwang ATTORNEY B. B. JACOBSEN ET AL Sept. 23, 1952 l ELECTRIC wA'vE FILTER 4 Sheets-Sheet 2 Filed Jan. 5, 1946 Sept. 23, 1952 B. B. JAcoBsl-:N ETAL ELECTRIC WAVE FILTER 4 Sheets-Sheet 5 Filed Jan. 5, 1946 A wm.

.if N m T A ELECTRIC WAVE FILTER 4 Sheets-Sheet 4 Filed Jan. 5, 1946 ATTORNEY Patented Sept. 23, 1952 2,611,874. `ELECTRIC WAVE FILTER Bent Bulow Jacobsen and William John Mitchell,

London, England, assignors, by mesne assignments, to International Standard Electric Corporation, NewA York, N. Y., a `corporation of Delaware Application January 5, 1946, serial No. 639,289 In Great Britain January 1,1945

Section 1, `liublic Law 690, August 8, 1946 Patentexpires January 1, 1.965

s'oiaims. (ci. 25o- 36) transformers (wh-ich are `equivalent to filters)` thislarge phase change is often the cause of oscillation atl undesired frequencies] An' Voscillator which filters according to the present'invention-may be appliedwith advantage is of the kind'described in British Patent Specification No.V 510,379. vThis is known as the bridge stabilised type of oscillator, and comprises a feedback path including a Wheatstone bridge containing a vresonant circuitfin" one arm and athermal resistance element in another. The feedback path is coupled to the amplifier path by two tuned transformers, and it has been found that the transformers introduce 'large phase changes which produce spurious oscillation frequencies.

It is veryundesirable to introduce reactance elements into the bridge for the purpose of preventing oscillations at undesired frequencies, since the stability at the desired frequency would be impaired.

` One well known method of avoiding this trou-y ble involves the lintroduction of a phase compeneating network'in-theamplifier. The use of coupling filters according i to` vthe" present invention oiers a'ncther and in somev respects' a better solution, Vthoughit maylbeusedif desired in addition to the'known'm'ethods.

It will be understood, `of course, that this is not theonly possible application of the filters according to the-presentV invention. They may be used wherever their `special properties can be usefully employed j I Accordingto theinvention' there is provided an electricwave,selective network comprising an electric wave filter terminated at one end by a. resistance, and by-path means associated therewith 'and ,sc disposed that when an alternating voltage 4is applied to the input ofthe network a voltage is obtained at the output of the network which is a combination of a voltage proportional to the inputvoltage, and a`voltagevproportional to the voltage at the output of the'saidfllter.

, 2 y, ,3 According to another aspect, the invention provides an electric wave selectivenetwork comprising an electric wave filterterminated at oney end by a resistance,l and by-path means vassociated therewith and adapted to reduce the phase change suffered Yby waves transmitted through the network so that the said phase change is substantially less than at any frequency.

The invention will be described with reference to the accompanying drawings in whichz' Fig. 1 shows a'block schematic diagramof a selective network according to the invention;

Figs. 2 and 3 show schematic circuit diagrams of two embodiments; l

Figs. 4 and 5 show respectively therattenuation-frequency characteristics and the correspending phase-change-frequency characteristics for a selective network according to the invention; and *Y Fig. 6 shows a schematic circuit diagram vofan oscillator employing a selective lnetworkaccording to the invention.

In this specification reference will be made to:

output a certain fraction or multiple of the volt` age applied at the input of the filter 5'. The outputs of the blocks 5 and `6 are connected to a'mixing device l adapted to produce at the output' terminals 3 and 4 a voltage proportionalfto the sum or difference of the voltages at the outputs ofl blocks'l and 6. -The device 6 may not conta-in fany actual apparatus, and may "consist simply of' two direct conductors, or it may be anv attenuator, transformer, or another filter; orqit may contain an amplifier. Likewise the device 1 may contain a mixing valve or valves'for example, or other apparatus, or again it may be simply connecting conductors as indicated by the dotted lines.

It will be seen that the device lforms a bypath tothe output for a proportion of theinput voltage. For this reason the type of lter .ac- (fix'ding to the invention will be called a by-path er. [Y v Provided there are no one-way devices,y such as valves, in anyl of the blocks 5, 6 or 1, then'as is well known, the terminals 3 and 4 can be used as input terminals and I and 2` as output Vterminals, and the by-path filter will have the same transmission properties.

Fig. 2 shows the details of a simple by-path filter according to the invention. It comprises a transformer 8 series tuned on the input side by a condenser 9, and parallel tuned on ythe output side by a condenser I0. The transformer 8 is terminated on the output side by a load resistance Rz designated I I, and the resistance I2 represents the input load connected to the terminals I, 2. A generator I3 is shown connected in series with the resistance I2 to represent the source of the waves applied to the filter. The resistance I4 represents the resistance component r of the primary winding of the transformer 8. The total resistance of the elements I2 and I4 is R1, and R2 includes the shunt resistive component of the tuned secondary winding of the transformer 8.

The transformer 8 together with the condensers 9 and I0 is equivalent to a half section of a band pass filter of type IVIcshoWn in Fig. 168A of the above-identified textbookV Transmission Networks and Wave Filters by T. E. Shea, with an ideal transformer with a step-up ratio of p2, where @2:R2/R1. This may .be shown by a series of transformations of the network elements according to well known principles, as set out .for example in the articles by E. K. Sandeman in the Wireless Engineer September and October, 1941. The filter constituted by the elements 8, 9, I and I4 will be called the prototype filter for convenience.

One terminal of the secondary winding of the transformer 8 is connected to the output terminal 3 and the other'terminal is connected by a conductor I to the input terminal I. The output terminal 4 is connected to the input terminal 2 by the conductor I5.

The inductances of the transformer windings and the capacities of the condensers 9 and I0 should be chosen according to formulae which will be given later so that the prototype ilteris correctly terminated at its terminals by the loads R1 and R2 respectively. It will be seen that the output voltage at the terminals 3and 4 is equal to the sum ofthe voltage across the load II and the voltage across the input terminals I, 2.

In this particular form of the by-path lter it is assumed that the output load to which the terminals 3 and 4 are to be'connected is of substantially infinite impedance, such as'the control grid circuit of a valve.

At the mid-band frequency of the filter, the reactance of the filter as seen from the terminals I, 2 will be zero, and if 2E is the electromotive force of the generator I3 then the voltage contributed by the by-path will be E (T+R1) /R1, while the output voltage of the prototype filter developedacross the resistance R2 Will be gaE.

The `design formulae for the by-path filter of Fig. 2 will now be given. AThe following list gives definitions of the symbols used:

frthe frequency variable f1 and fz--Ylower and upper cut-off frequencies fmzmid-b'and frequencytvfijz w, wi, w2,wm":`21f, 21rf 1, 21rf2, 21rfm,.lSpC5Ve1y Ri'zinput image `impedance of. prototype filter at .fm Rzzoutput image impedance of prototype-filter at m p2:R2/ R1 Y P=primary Winding inductance of transformer 8 szsecondary winding inductance of transformer 8 kzicoupling factor of transformer 8 M :mutual inductance of transformer 8 :WPS

C1=capacity of condenser 9 Czzcapacity of condenser I0 `As explained in Sandemans article, for example, it can be shown that for the design of the prototype filter,

The discrimination factor of a filter is the factor which determines the practical performance of the filter, and is defined as the vector ratio of the output voltage at the terminals 3 and 4 at the frequency fm, to the output voltage at any other frequency f, itbeing assumed that the input voltage at terminals I and 2 is the same at both frequencies. The discrimination is the logarithm of the discrimination factor, and the real part of this logarithm is also equalto the difference between the attenuation through the filter at the frequency f and the attenuation at the mid-band frequency fm, this difference being preferably expressed in decibels. Thus in an ideal band-pass filter the magnitude of the discrimination factor would be equal to 1 over the pass band, and would be infinite outside the pass band. Such a condition is, of course, never attainable in practical filters.

Now it can be shown that the discrimination factor of the by path filter is equal to l/(a-i-y'b) This is perhaps most easily shown by expressing the-prototype filter in the form of the corresponding 'half sectionvtype IVIC. assuming that the prototype filter isterminated in the manner explained, and then working out the output/ input voltage,ratio'at'termnals 3, 4 and I, 2 for any value of In this calculation, the output voltage is, of course, the sum of the normal outputv voltage-of the prototype lter and the input voltage at terminals I, 2. By putting .fr-:0 the corresponding value of the ratio is obtained for the mid-band frequency. The ratio of the two values so obtained is the required discrimination factor, and it will be 'found more convenient to work out the' reciprocal of the discrimination factor in the form a-l-'fb as indicated above.

The magnitude of the di'scriminationfactor is then l/\/a2-|b2 and .the phase angle 0=tan1 b/a.

By putting a:?= it can be shown that the discrimination measured in decibels tends to the asymptotic value 20 login (tb-l-Z-p) on either side of the pass band, vand the phase change tends to zero instead of to plus or minus 180.

The curves shown in Figs. 4 and 5 .illustrate the results obtained in a particular case of Fig. 2. The following were the design requirements:

Fig. 4 gives the discrimination and Fig. 5 the corresponding phase change. In each of these figures the curve designated II refers tok the prototype transformerlter without the by-path feature, and that designated I8 refers to the bypath filter according to Fig. 2. The line I9 in Fig. 4 gives the asymptotic value'of the discrimination. It will be seen from Fig. 5 curve I1 that while the phase change for the prototypev filter rapidly approaches closely to i 180, that of the bypath lter (curve I8) does not exceed about 127 anywhere, and tends ultimately to zero on either side of the band. The curves I1 and. I8 of Fig. 4 show that in the by-path filter the out off has been somewhat sharpened at the expense of a relatively low asymptotic discrimination of about 221/2 db.

The other curves shown in Figs. 4 and 5 will be explained later.

Referring again to Fig. 2, it will be understood that the proportion of the input voltage transferred to the output by the by-path may be varied in a number of ways, and its sign may be r-eversed.

Thus a transformer (not shown) may be interposed in the conductors I5 and IB by which a step-up or step-down may be produced. Alternatively, an attenuator, or amplifier, may be included, or another filter adapted to pass at least those frequencies where a phase change reduction is required. Any combination of such elements might be used. It will be understood that the particular numerical case for which the curves of Figs. 4 and 5 have been drawn is given to illustrate the design of a by-path filter according to the invention, and that filters meeting other requirements can be designed in a similar manner. The amount of phase change reduction obtained depends on Ip and p, which should be chosen accordingly. 'Ihe greater the reduction required the smaller will be the asymptotic discrimination.

The filter of Fig. 2 is intended to work into a substantially infinite impedance such as the control grid circuit or" a valve, lwhich will be connected to terminals 3 and 4. The lter could also be used in the opposite direction. Thus the terminals 3 and 4 could be connected to a high impedance source such as the anode circuit of a pentode valve, and would then deliver the waves to a circuit of impedance Ri-r.

If it is desired to work the by-pathfilter between two impedances, neither of which is substantially infinite, the arrangement yor Fig. 3 may be used. In this figure those elements which are the same as in Fig. 2 have been given the same designations and will not again be described. The addition of the two voltages is effected by a valve having two control grids, one of which is connected direct to terminal l, and the other to the upper endof the secondary winding of the transformer 8. The rlower ends of the two windings are connected together by the conductor 2| and are grounded. The cathode of the valve 20 is earthed through a conventional bias net- Work 22 and the anode is connected through the primary Winding of a transformer 23 to the high tension source 24 which is shunted by a by-pass condenser 25. The secondary winding -of the transformer 23 is connected to the output terminals 3 and 4. A high resistance grid leak 25 is provided for the first mentioned control grid.

It is obvious that the output current of the valve 20 will be` proportional to the vector sum ofthe voltages applied to the two control grids. One of these voltages is proportional to the output voltage of the prototype filter, and the other is proportionalto the input voltage. rI'he ratio of vthe transformer 23 may evidently be designed to match the impedance of any desired output load connected to terminals 3 and 4.

In order to vary conveniently the proportions of the voltages applied to the two grids, these grids may be connected, for example, to adjustable taps (not shown) on the respective resistances I I and 26 instead of as shown in Fig. 3. It is obviousthat the valve 2G and its associated circuit rnay be varied in a number of well known Ways. If there is a direct current path through the input load I2, the leak resistance 25 would not be necessary and could be omitted. The valve should, however, be biassed so that it operates normally. If the grid nearerl to the anode is a screen grid, it could be' polarized positively through a suitable resistance (not shown) in the usual way. As an alternative arrangement the addition of the two voltages might be effected by two separate valves operating into a common load.

It is emphasised that the prototype filter need not be of the type IWC; it need not contain an actual transformer at all,.and is not necessarily a transforming filter or even a band-pass lter.

The by-path principle can be applied to any electric wave filter for the purpose of reducing or modifying the phase change produced thereby.

Fig. 6 shows one example of the application of a by-path filter according to the invention. It is employed in a bridge stabilised oscillator of the kind. described in patent specification No. 510,379. Fig. 6 is a re-drawing of Fig. 1 of that specification in order to show how the bly-path lter may be included. The by-path filter is shown in the dashed outline 2'! and includes the elements 8, 9, IIJ, II, I4, I5 and I6 described above with reference to Fig. 2. The by-path filter connects one pair of diagonal corners of the bridge 23 to the control grid circuit of the valve 29, the anode circuit of which is connected to the other pair of diagonal corners of the bridge 28 through a transformer 30 having a condenser 3l shunting the primary winding and condensers E2 and 33 in series with the balanced secondary winding. The resistance 34 represents the output circuit to which the oscillations are delivered.

The elements 3B, 3|, 32 and 33 constitute a half section transforming band pass filter of type IVIC and should preferably be designed in the manner to be explained presently. The valve 29 with its associated circuit and the bridge 28 are arranged substantially in the same way as eX- plained with reference to Fig. 1 of specification No. 510,379 and the circuit as a whole operates similarly except for the reduction in the phase change which is brought about by the use of the by-path filter according to the invention.

Assuming, for example, that the circuit is designed to oscillate at 124 kilocycles per second, the by-path filter 21 may be designed to meet the requirements specified in the numerical example given above, to which the curves of Figs. 4 and 5 7 apply. The elements 30, 3l and 32 should preferably be chosen according to the-formulae .given above for the prototype filter to producea rather wider band than the by-path filter also centred on 124 kcs.

For example B may be taken .as 6.5. The value of o will be decided -by theimpedances between which the filter has to Work. The attenuation and phase change curves for this filter are designated 35 in Figs. 4 and 5. The curves representing the combined effect of the two filters are designated 36. Referring to Fig. 5 it wiil be seen from curve 36 that the phase change does not exceed about 162 in the neighborhood of the pass band, and never reaches 180 at any frequency. This means that the total phase change through the filter and amplifier circuit cannot approach near to an odd multiple of vr except at the mid-band frequency, which is the desired frequency of oscillation. It is therefore possible to increase the gain of the amplifier indefinitely without causing oscillation at undesired frequencies. Reference to the curves Il and 35 of Fig. 5 shows that with two prototype filters, the total phase change for the two filters will be equal to 1r in the neighbourhood of the cut olf frequency of the narrower band filter (at about 132 kos), with the result that the circuit may oscillate at the wrong frequency if high gains a-re used.

When applying the principle of this invention to an oscillator, the transformers or filters should preferably be designed for different band widths and the by-path principle should preferably be applied to the one with the narrower band. In Fig. 6, the two lters could have been interchanged so that the by-path lter is at the output of the valve instead of at the input. Actually both the filters could be by-path filters according to the invention if desired.

What is claimed is z 1. A frequency selective network comprising a rst pair of terminals, a second pair of terminals, a pair of parallel vwave transmission paths connectingthe first and second pair-s of terminals, one of said paths being by itself substantially direct and non-frequency discriminating, the other path comprising a frequency band transmission filter, a resistance Vterminating said iilter and common to both pathsto limit the phase change of waves transmitted through the paths to said second pair of terminals to less than 180 degrees at `any frequency, said filter including a rst condenser and the primary winding of a transformer forming a series tuned combination across the first pair of terminals, and said resistance vbeing shunted across Jthe secondary winding of said transformertogether with acondenser to provide a parallel tuned combination across the second pair of terminals.

2. A frequency selective network according `to claim 1, in which both of said combinations are tuned to the mid-frequency of the band of Ifrequencies to which the vnetwork selectively transmits.

3. A frequency selective network comprising a first pair of terminals, a second pair of terminals, a pair of parallel wave transmission paths connecting the rst and vsecond pairs of terminals, one of said paths being Iby itself substantially direct and non-frequency discriminating, the other path comprising a frequency band transmission lter. a resistance terminating said filter and common to both paths to limit the phase change of waves transmitted through the paths to said second pair of terminals to less than degrees at 'any frequency, said second set of terminals :being connected across the input circuit of a grid-controlled electron tube, the output of said electron tube being coupled through a highly selective additional network to the first pair of terminals.

4. A frequency selective network according to claim 3, in which said additional network comprises a bridge having two sets of conjugate points, one set of conjugate po-ints being connected to the output of said electron tube, and the other set of conjugate points being connected to said first pair of terminals.

5. A frequency selective network according to claim 4, in which said bridge has in one arm thereof means highly resonant to a particular frequency and in another arm thereof means including a temperature dependent resistance.

BENT BULOW JACO'BSEN. WILLIAM JOHN MITCHELL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,759,952 McCurdy May 27, 1930 1,902,031 Holden Mar. 21, 1933 1,968,237 Weyers July 31, 1934 2,031,100 Engel Feb. 18, 1936 2,163,403 Meacham June 20, 1939 2,240,450 Wolfskill Apr. 29, 1941 2,261,286 Rankin Nov. 4,1941 

